The present invention relates to a magnetic drive device for driving a rotor in rotation and also axially while outside a cylindrical wall, such as a vacuum shield, having the rotor disposed therein, so that the drive device is magnetically out of contact with the rotor. The invention also relates, for example, to such a magnetic drive device for use in semiconductor production apparatus for handling wafers in a vacuum.
Magnetic drive devices are known which are adapted to drive a rotor which is held out of contact therewith by a cylindrical wall and which comprise two movable housings provided outside the cylindrical wall axially movably, a magnetic bearing disposed in one of the housings for holding the rotor radially and axially thereof, and a magnetic coupling having a drive portion provided in the other housing for transmitting a torque to the rotor.
With the magnetic drive device, the rotor is rotated by rotating the drive portion of the magnetic coupling and is moved axially by axially moving the housings owing to the axial holding force of the magnetic bearing.
However, the following problem is encountered when the magnetic drive device described is used for driving the rotor disposed inside the cylindrical wall.
The magnetic bearing generally has a small air gap and is made movable along the cylindrical wall. The cylindrical wall must therefore be reduced in thickness and machined with high accuracy. However since the above drive device has two housings which are movable along the cylindrical wall, the wall needs to have an increased length. Accordingly, extreme difficulties are encountered in making such a cylindrical wall with a reduced wall thickness and in achieving, high by machining.
For this reason, the magnetic bearing to be used actually has a large air gap. However this reduces the holding force of the bearing especially in the axial direction, consequently necessitating an increased control current and therefore a magnetic bearing of larger size to obtain a sufficient axial holding force.
We have already proposed a magnetic drive device which comprises a rotor disposed inside a cylindrical wall so as to be rotatable and axially movable, two magnetic bearings fixedly provided on the cylindrical wall toward one end thereof for radially holding the rotor at two portions spaced apart axially thereof, and a magnetic coupling having a drive portion disposed around the cylindrical wall toward the other end thereof so as to be rotatable and axially movable for transmitting a torque and an axial holding force to the rotor (see Japanese Patent Application HEI 3-30913).
With the proposed magnetic drive device, however, the two magnetic bearings for holding the rotor radially thereof are arranged toward one end of the cylindrical wall and axially spaced apart by a small distance. This entails the problem that the rotor is dynamically in poor balance and is liable to incline, thereby adversely affecting the control or the magnetic bearings.